A rare earth sintered magnet is manufactured by pulverizing a rare earth magnetic alloy (material alloy) into alloy powder, compacting the alloy powder to obtain a green compact, sintering the green compact and subjecting the sintered body to an aging treatment, machining and other such processes. Currently, as rare earth sintered magnets, two types of magnets, rare earth-cobalt type magnets and rare earth-iron-boron type magnets, are used extensively in various fields of applications. Among others, rare earth-iron-boron type magnets (which will be referred to herein as “R—Fe—B type magnets” wherein R is at least one type of element selected from the group consisting of rare earth elements and yttrium, Fe is iron, and B is boron) are used more and more often in various types of electronic appliances because the R—Fe—B type magnets exhibit a magnetic energy product that is higher than any of other various types of magnets and yet are relatively inexpensive. A transition metal element such as Co may be substituted for a portion of Fe. Also, carbon may be substituted for up to one half of boron.
To manufacture a sintered magnet having a desired shape, R—Fe—B type rare earth magnet powder (i.e., rare earth alloy powder) is first compacted with a press, to obtain a green compact having a size that is larger than that of a final magnet product. The green compact is sintered, and the resultant sintered body is ground or cut with a cemented carbide saw blade, a rotary grindstone or the like to render the sintered body to have a desired shape. For example, a sintered body is first manufactured as a block, and the block is then sliced with a saw blade or the like to obtain a plurality of sintered plates.
A sintered body of a rare earth alloy magnet such as a R—Fe—B type magnet is very rigid and brittle and has a large machining load. Therefore, high-precision grinding of such a sintered body requires hard work and takes long time. For this reason, the machining process is a major cause of increases in the production cost and time thereof.
To solve the above problem, grinding of a green compact before sintering has been proposed, for example, in Japanese Laid-Open Patent Publication No. 8-64451 and No. 8-181028.
Japanese Laid-Open Patent Publication No. 8-64451 discloses a technique of chamfering a green compact for a bow ferrite magnet with a rotary grindstone or a rotary brush. If this technique is applied to a green compact for a R—Fe—B type magnet that is highly susceptible to oxidation, the following problem may occur. Friction heat is generated between a rotary grindstone or a rotary brush and the green compact, and this may cause rapid reaction of a rare earth element and iron in the green compact with oxygen and water in the atmosphere. As a result, ignition of the green compact may possibly occur in the worse case. Even if the worst case is avoided, the magnetic properties of the magnet will be deteriorated.
Japanese Laid-Open Patent Publication No. 8-181028 discloses a technique of preventing oxidation of a green compact during machining thereof, in which the green compact is immersed in mineral oil, synthetic oil or plant oil, and is cut with a rotating blade in the immersed state.
This technique indispensably requires a process step for removing the mineral oil or other substances from the green compact after the cutting and before sintering. If degreasing is insufficient, carbon contained in the oil acts as an impurity in the sintering process, and this deteriorates the magnetic properties.
In addition, in the cutting with a saw blade and the like, a large cutting clearance is required for the green compact, and this decreases the yield of the material.